Scott Swing let me know that the engine mount for a TCM IO-550 is 24″ wide at the bottom, and that the bolts penetrate the firewall just above the fuselage floor. I had previously cut down my header tank to make more room in this area, and I was relieved to find that there appears to be sufficient space to accommodate the bolts and the large washers. The 12″ mark is aligned with the fuselage centerline, and the tape measure shows the approximate location where the holes will be.
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